/Engine/Source/Runtime/Engine/Classes/Materials/MaterialInstanceDynamic.h
Use this function to set an initial value and fetch the index for use in the following function.
bool InitializeVectorParameterAndGetIndex ( const [FName](API\Runtime\Core\UObject\FName) & ParameterName, const [FLinearColor](API\Runtime\Core\Math\FLinearColor) & Value, int32 & OutParameterIndex )